6.: Anhydrite Swelling

Many carbonate reservoirs are associated with overlying salt pans or updip sabkhas that can contain gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O). As the rock undergoes the burial process over geological time, it is possible that the water molecules in the gypsum structure will be removed, thereby converting the gypsum into anhydrite:

Alternatively, anhydrite can form through the evaporation of seawater. Massive accumulations of anhydrite occur when salt domes form a caprock. Anhydrite is typically 1 to 3% of the salt in salt domes and is generally left as a discrete layer when the halite is removed by pore waters, so the cap to a carbonate sequence is often anhydrite.